At present exhaust fumes from the engine of a vehicle flow through an exhaust pipe and through a muffler to be exhausted. As exhaust fumes generally contain a large amount of incompletely burned gasoline and carbon monoxide, which is toxic to human bodies laws are enacted to limit toxic gas emission to protect human health and the environment.
However, vehicles can produce toxic gas in exhaust fumes of a greater amount than the law allows after they are used for a certain period of time.